Effects of logging on the spawning bed environment in two Oregon coastal streams

Abstract:

The effects of two patterns of logging on the intragravel environment were studied in three Oregon coastal streams between June 1968 and June 1969. The watershed of one stream (Needle Branch) had been clearcut, and that of a second stream (Deer Creek) cut in staggered settings in 1966. A third watershed (Flynn Creek) served as an unlogged control. The dissolved oxygen content, biochemical oxygen demand, and temperature of the intragravel water were determined, as well as the size composition and organic content of the gravel. Changes were evaluated in terms of their effects on the survival of salmonid eggs and alevins. Water samples for analysis of dissolved oxygen and biochemical oxygen demand were removed from the streambed by means of standpipes. Intragravel water temperature was obtained with a thermister probe forced into the redds and with thermograph probes buried at selected locations in the streambed. Gravel samples were removed from recent redds and from previously used spawning sites ("former redds") with a newly-developed sampler that incorporated dry ice and acetone to freeze an intact core of gravel around an iron pipe. Samples were cut into strata 8.3 cm in depth and analyzed for size composition by volumetric displacement and for organic content by weight loss upon ignition. Dissolved oxygen in redds of Needle Branch averaged 7.15 mg/1, whereas that in Deer Creek averaged 8.91 mg/1 during 1969. Oxygen levels in Needle Branch redds in 1969 were 37.4 percent lower than those reported in 1964. Oxygen in Deer Creek redds dropped 12.7 percent in the same period. Dissolved oxygen at permanent standpipe locations was significantly lower than that in redds and showed greater variability. Oxygen levels were positively correlated with streamflow and negatively correlated with temperature. Organic content of the gravel ranged from 0.33 to 7.52 percent by weight; less than 3 percent of the organic material was larger than 6.35 mm. The quantity of organic material was directly related to the amount of fine sediment in the sample. Recent redds in Needle Branch contained significantly less organic debris than did former redds. However, the organic content of redds in Needle Branch did not differ statistically from that in Deer and Flynn Creeks. The biochemical oxygen demand of the intragravel water averaged 1.95 mg/1 for the three streams; differences among streams were not statistically significant. Stratification of fine sediment was evident in many redds, but a definite pattern of stratification could not be detected. Gravel size composition in Needle Branch did not differ statistically from that of the other streams. Recent redds in Needle Branch contained significantly less sediment than did former redds. Intragravel water temperature lagged from 2 to 6 hours behind surface temperature in attaining the diurnal maximum. Water temperature decreased with depth in the gravel in Needle Branch and Deer Creek on clear days, but the intragravel water was almost isothermal in Flynn Creek. Fluctuation in intragravel water temperature occurred as early as March, and maxima as great as 19.7°C were recorded prior to complete emergence of coho salmon. Surface and intragravel temperatures reflected the amount of shade over the stream surface. Survival to emergence of coho salmon appeared to be little affected by the observed changes in the intragravel environment.